1. Technical Field
The present invention relates to a screw compressor including meshing male and female screw rotors rotating about two axes parallel to each other, and defining a working space in cooperation with a casing, and, especially, to a screw compressor of oil-free type which requires no oil to be supplied to the working space.
2. Relative Art
In a screw fluid machine treating a gas, such as a compressor, an expander, a vacuum pump and so on, the temperature of the gas at the high pressure side becomes high and, generally, for example, as high as 300.degree. C. in a screw compressor of oil-free type having a gas compression ratio 8. As a result, the thermal expansion of the rotors becomes a considerable amount. For solving this problem in, for example, U.S. Pat. No. 4,475,878, the amount of clearance between the rotors is determined taking into consideration this thermal expansion so that the rotors do not contact each other even in operating condition of the compressor. Further, heretofore, for suppressing errors in estimating thermal expansion amount and for assuring the strength of the material and the reliability of the bearing parts, several measures have been attempted such as, for example, cooling the compressed gas by providing a water jacket in the casing, or the rotors by passing a cooling oil through axial holes formed in the central portions of the rotors.